Method for cascading detachable conditional access modules, circuit for inserting a predetermined sequence and circuit for detecting the said sequence in order to implement the method

1. A circuit comprising: (a) a comparator having a first input for receiving a data stream and a second input for receiving data corresponding to a predetermined sequence, said data stream comprising packets of fixed length separated by an interpacket space of fixed length, said interpacket space comprising said predetermined sequence; (b) a data generator for generating said data corresponding to said predetermined sequence, said data generator comprising a memory for storing a table containing, at successive addresses, said data of said predetermined sequence, an address input of said memory being set to a first address each time said comparator detects inequality and said address input of said memory is incremented or decremented each time the comparator detects equality, said table being associated with means for sending a first pulse (N>16) when all the data corresponding to said predetermined sequence has been sent; and (c) a pulse generator for generating a second pulse indicating the length of said packet, said pulse generating means being enabled in response to detection of said first pulse indicating that all the data corresponding to said predetermined sequence has been sent.

2. A circuit according to claim 1 wherein said address input of said memory is given by a counter ( 111 ) reset to 1 at the initial address each time the comparator detects inequality, and incremented or decremented each thime the comparator detects equality, the output of the comparator addressing said memory and the counter being incremented or decremented in a range of limited values and sending a pulse at the end of the range (N>16).

3. Circuit according to claim 2 , characterised in that the means for generating the data of the predetermined sequence consist of a counter whose output directly gives the values of the octet of the predetermined sequence.

4. Circuit according to claim 1 , characterised in that the means ( 113 ) for generating a pulse corresponding to the length of a packet consists of a monostable multivibrator which cannot be reset to zero.

5. A circuit for inserting a constant predetermined sequence in a data stream having packets which are separated by interpacket spaces, the circuit comprising: (a) means for generating said constant predetermined sequence, said predetermined sequence comprising different data concatenated together, said means including a counter reset to the initial value of the predetermined sequence at each end of packet and incremented by a predetermined amount at each octet clock cycle; (b) a multiplexer having a first input for receiving said data stream and a second input connected to said generating means for receiving said data corresponding to said predetermined sequence; and (c) a detection circuit for detecting a start and an end of a packet in said data stream and for sending data selecting one of said first and second inputs.

6. The circuit according to claim 5 , wherein said counter is incremented by 1 at each octet clock cycle.